advanced sql.pdf

8/11/2019 advanced sql.pdf

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SQL - 3

Week 7 - 2


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2

Aggregate operators

What is aggregation?

Computing arithmetic expressions, such as

Minimumor Maximum

The aggregate operators supported by SQL are:

COUNT, SUM, AVG, MIN, MAX


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Aggregate Operators

COUNT(A): The number of values in the column A

SUM(A): The sum of all values in column A AVG(A): The average of all values in column A

MAX(A): The maximum value in column A

MIN(A): The minimum value in column A

(We can use DISTINCT with COUNT, SUM and AVG to compute

only over non-duplicated columns)


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Using the COUNT operator

Count the number of sailors

SELECT COUNT (*)

FROM Sailors S;


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Another Aggregate Query

Count the number of different sailor names

SELECT COUNT (DISTINCT S.sname)

FROM Sailors S;


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Example of SUM operator

Find the sum of ages of all sailors with a rating of 10

SELECT SUM (S.age)

FROM Sailors S

WHERE S.rating=10;


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Example of AVG operator

Find the average age of all sailors with rating 10

SELECT AVG (S.age)

FROM Sailors S

WHERE S.rating=10;


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Example of MAX operator

Find the name and age of the oldest sailor

SELECT S.sname, MAX(S.age)

FROM Sailors S;

But this is illegal in SQL!!


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Correct SQL Query for MAX

SELECT S.sname, S.ageFROM Sailors S

WHERE S.age = ( SELECT MAX(S2.age)

FROM Sailors S2 );


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Alternatively

SELECT S.sname, S.ageFROM Sailors S

WHERE ROWNUM


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Banking Examples

branch (branch-id, branch-city, assets)

customer (customer-id, customer-name, customer-city)

account (account-number, branch-id, balance)

loan (loan-number, branch-id, amount)

depositor (customer-id, account-number)

borrower (customer-id, loan-number)


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INExample 1

Find the account numbers opened at branches of the bank in

Fairfax

SELECT A.account-number

FROM account A

WHERE A.branch-id IN(SELECT B.branch-id

FROM branch B

WHERE B.branch-city=

Fairfax

)


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INExample 2

Find the account numbers opened at branches 101 and

102 of the bank


FROM A.account

WHERE A.branch-id IN(

101

,

102

)


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EXISTS

TheEXISTSpredicate is TRUE if and only if the Subquery

returns a non-empty set.

TheNOT EXISTSpredicate is TRUE if and only if the

Subquery returns an empty set.

TheNOT EXISTScan be used to implement the SETDIFFERENCE operator from relational algebra.


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EXISTSExample 1Select all the account balances where the account has been

opened in a branch in Fairfax

SELECT A.account-balance

FROM account A

WHERE EXISTS(SELECT *

FROM branch B


Fairfax

AND B.branch-id=A.branch-id)


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EXISTSExample 2

Select all the account balances where the account has not been

opened in a Fairfax branch

SELECT A.account-balance

FROM account A

WHERE NOT EXISTS(SELECT *

FROM branch B


Fairfax

AND B.branch-id=A.branch-id)


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Quantified Comparison PredicateExample 1

Select account numbers of the accounts with the minimum

balance


FROM account A

WHERE A.balance


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Aggregate Functions in SQL

revisitedSQL provides five built-in aggregate functions that operate

on sets of column values in tables:

COUNT( ), MAX( ), MIN( ), SUM( ), AVG( ).

With the exception of COUNT( ), these set functions must

operate on sets that consist of simple values-that is, sets of

numbers or sets of character strings, rather than sets of rows

with multiple values.


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Example 1

Select the total amount of balance of the account in

branches located in Fairfax

SELECT SUM(A.balance) AS total_amount

FROM account A, branch B


Fairfax

AND

B.branch-id= A.branch-id


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Example 2Select the total number of opened accounts

SELECT COUNT(A.account-number)

FROM account AOR

SELECT COUNT(*)

FROM account

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